The present invention relates to a method and apparatus for anchoring corrosion coupons downhole in the production tubing of a producing oil and/or gas well. More particularly, the present invention is directed to a method and apparatus which makes it possible to closely duplicate the actual conditions experienced by the production tubing to enable a more accurate evaluation of corrosion rate and, hence, operative life of the tubing, to be made.
Monitoring the corrosive effects of well fluids using corrosion coupons allows (1) a determination of the corrosion problem downhole to be made in the actual environment (pressure, temperature, etc.) to which the production tubing is exposed, (2) an evaluation of the effectiveness of the corrosion mitigation program to be conducted, and (3) an assessment of the life of the production tubing to be made.
Failure to monitor corrosion rates and to take appropriate measure to slow those rates can reduce the operative life of the well. Accordingly, monitoring the corrosive effects of the corrosives entrained in the well fluids (such as CO.sub.2, H.sub.2 S and O.sub.2) is becoming increasingly important. A number of coupon holders for positioning corrosion coupons downhole are currently available on the market today. The prior art systems of which applicant is aware suffer from one or more of the following defects: (1) the coupon holder can only be used where the well is capable of accomodating specially configured tubing (i.e., a side-pocket mandrel); (2) the conditions (pressure, flow rate, etc.) experienced by the coupon are not representative of those experienced by the tubing--thus, the corrosion rate may also differ; (3) the anchoring technique within the tubing may be unreliable and/or potentially harmful to the interior surface of the tubing; (4) the manner of securing the coupon in the holder is such that it effectively alters the corrosion rate impairing the accuracy of the monitoring system; and (5) the holder may unduly restrict the flow of fluids therethrough, which both reduces the rate of recovery of fluids and alters the flow and corrosion conditions experienced by the coupon (i.e., gives a faulty indication of corrosion rate).
The present invention overcomes these deficiencies of the prior art. A preferably cylindrical coupon is anchored in one or more predetermined locations within the production tubing using state-of-the-art nipples and locking mandrels. The holder which houses the coupon is threadingly attached to an adaptor which, in turn, is threaded onto the end of the locking mandrel. The coupon is maintained in position within the cylindrical housing by end retainers which telescopically receive the ends of the coupon and which are compressively engaged by the end of the housing and the end of the adaptor. This compressive force is sufficient to firmly engage the coupon but is not excessive to the point of deforming the coupon or the end retainers. The amount of compressive force exerted is limited by metal to metal engagement between the adapter and the coupon holder. Further, the metallic coupons are maintained in positioned out of contact with any other metallic surface or object.
Preferably, two or more coupons are used. In this manner, it is possible to cross-check the corrosion experienced by each. The two coupons can be made of the same material for purposes of cross-checking or they may be made of different materials in order to determine how different alloys are affected by the same environment. With two coupons, a center retainer is telescoped over the adjoining ends of the coupons to maintain proper alignment. The minimum dimension inside the housing occupied by the coupon is not less than the minimum internal dimension of the locking mandrel so as to minimize the impediment of flow caused by the holder.
Accordingly, the coupon holder of the present invention provides a corrosion monitoring system that (1) is firmly anchored in position, (2) does not damage the internal surface of the casing, (3) does not require specially configured tubing but, rather, utilizes a conventional nipple that is ordinarily positioned within the tubing string to permit other types of periodic downhole operations, (4) minimizes the obstruction of fluid flow, (5) substantially duplicates the flow conditions experienced by the actual tubing so as to provide an accurate reading of corrosion rate and, (6) is secured by means not requiring a penetrating metallic fastener or contact by any other metallic object which can unduly alter corrosion due to cathodic reaction or the resulting internal strain within the coupon and, (7) has a securing means that permit back-flow around the coupon to increase the surface area exposed to the corrosive fluids.
Various other features, characteristics and advantages of the present invention will become apparent after a reading of the following detailed description.